David Moyes has been named as struggling West Ham's new manager.
The Premier League club have announced the 54-year-old former Everton, Manchester United and Sunderland manager as successor to Slaven Bilic who was sacked on Monday.
Bilic had been expected to see out the season with the Hammers but he was dismissed by joint-chairmen David Sullivan and David Gold in the wake of the 4-1 home defeat by Liverpool on Saturday.
The club have not disclosed the length of Moyes' contract.
Sullivan told the club website: "I would like to welcome David to West Ham United.
"This is a unique position for David Gold and myself - it is the first time in almost eight years at West Ham United that we have appointed a new manager during the season.
"We need somebody with experience, knowledge of the Premier League and the players in it, and we believe David is the right man to turn things around and get the best out of the players at the club.
"He is highly regarded and respected within the game, and will bring fresh ideas, organisation and enthusiasm.
"He proved with Everton that he has great qualities and we feel that West Ham United is a club that will give David the platform to display those qualities again."
Moyes has been out of management since he resigned from Sunderland at the end of last season, having failed to save them from relegation.
His appointment has been met with scepticism by some Hammers fans following his disappointing spells with United and the Black Cats, as well as an underwhelming stint in Spain with Real Sociedad.
Nevertheless, Sullivan and Gold are backing Moyes to help shore up a leaky defence as well as getting the best out of big-money summer signings Marko Arnautovic and Javier Hernandez.
Club-record capture Arnautovic has been such a disappointment he was booed off during the recent home defeat by Brighton.
Despite the poor start to the season, Sullivan and Gold wanted Bilic to see out the three-year contract he signed in June 2015.
But the manner of their capitulation on Saturday evening prompted them to dispense with the Croatian's services, which they say they did with "heavy hearts".
Less than 24 later Moyes was meeting his new players for the first time at the club's Rush Green training ground. His first game in charge will be at Watford a week on Sunday.
"I'm really looking forward to meeting the supporters and being in the stadium with them," Moyes said in a first interview published on the club's Twitter feed.
"I'm looking forward to seeing them get right behind the team, and my team also. We need the support, we need everybody with us.
"It's a big job we have in hand now but I'm sure with everybody together we can get the right results between now and the end of the season."